The present invention relates to a method of operating a lower pressure column of a double column distillation unit in which mixture is distilled within the lower pressure column by contacting an ascending vapor phase with a descending liquid phase the use of mass transfer elements. More particularly, the present invention relates to such a method in which the ascending vapor phase is initiated through vaporization of liquid within down flow reboilers. Even more particularly, the present invention relates to such a method in which vaporization of the liquid is distributed between two or more down-flow reboilers to produce a predetermined liquid to vapor ratio and without recirculation of sump liquid back to the down-flow reboilers.
Various mixtures are distilled within distillation column units having a higher pressure distillation column operatively associated with a lower pressure distillation column in a heat transfer relationship. The mixture to be distilled is processed within the higher pressure column and then is further processed within the lower pressure column. A typical example of such distillation is the low temperature rectification of air. Air is rectified by cooling a stream of compressed and purified air to a temperature suitable for its rectification. The air stream after having been cooled is introduced into a higher pressure distillation column to produce a nitrogen tower overhead and an oxygen-enriched liquid column bottoms. The liquid column bottoms introduced into a lower pressure column for further processing to produce a nitrogen rich tower overhead and an oxygen-rich liquid column bottoms. The oxygen-rich column bottoms can be taken as a liquid oxygen product or a vapor product that is pumped to a delivery pressure before being vaporized.
In both the higher and lower pressure columns, ascending vapor phases and descending liquid phases of the mixture to be distilled are produced. These vapor and liquid phases are contacted by mass transfer elements such as structured packing, random packing or trays. In the lower pressure column, nitrogen-rich vapor from the tower overhead region of the higher pressure column is condensed against the vaporization of the liquid column bottoms produced in the lower pressure column. The vaporization of the liquid in the lower pressure column initiates formation of the ascending vapor phase while the condensate formed by condensation of the nitrogen-rich vapor is recirculated back to both higher and lower pressure columns to initiate formation of the descending liquid phases. Alternatively a liquefying air stream can be used to vaporize the liquid in the low pressure column.
In order for the nitrogen-rich tower overhead to condense the liquid oxygen column bottoms, a temperature difference must be maintained. The narrowness of the temperature difference that can be maintained can depend upon the specific configuration or type of condenser-reboiler used within the lower pressure column. It has been found that down flow reboilers provide very narrow temperature difference. This is important because the lower the temperature difference to be maintained between the nitrogen to be condensed versus the oxygen to be vaporized, the lower the pressure within the higher pressure column and hence, the lower the pressure need be upon compression of the air.
The down flow reboilers that are used for such purposes are also referred to in the art as falling film devices in that a falling film of the liquid is initiated within heat exchange passages. As is well known in the art, down flow reboilers or falling film devices consist of a plurality of parallel plates to form the heat exchange passages. Generally, sheets of corrugated fin material are located within the heat exchange passages to increase the surface area of the heat exchanger and therefore the heat transfer capability of the heat exchanger.
The draw back in the use of down flow reboilers, particularly in the case of air separation, is that heavy contaminants such as hydrocarbons tend to concentrate in the liquid to be vaporized, for instance, liquid oxygen. Since a mixture of hydrocarbons and oxygen is potentially dangerous, such reboilers are operated to only vaporize part of the liquid, such as oxygen, to prevent dryout of the heat exchange passages and collection of hydrocarbons thereon. The remaining part of the liquid to be vaporized is supplied by recirculating sump liquid to the reboiler by pumping the sump liquid. The disadvantage of such operation is not only that a pump must be supplied, operated and powered, but also additional pumping energy imparted to the sump liquid must be compensated for by increased refrigeration and therefore increased power outlays in operating the plant.
As will be discussed, the present invention provides a method of operating a lower pressure column of a double column distillation unit in which all of the liquid is vaporized within down flow reboilers without the necessity of there being any pumping.